PD-1 is an inhibitory receptor induced in T cells after activation and regulates the balance between stimulatory and inhibitory signals that plays a vital role in the induction and maintenance of anergy and peripheral tolerance. By inhibiting T effector cell function, PD-1 signaling mediates tissue tolerance by suppressing tissue-reactive T cells and protects against immune-mediated tissue damage. PD-1 can also regulate T cell tolerance by promoting the generation of Treg cells. PD-1 requires proximity and co-ligation with the TCR, resulting in tyrosine phosphorylation of the ITSM motif in C-terminus of PD-1 cytoplasmic tail leading to the recruitment of SHP-2 and inhibition of multiple downstream activation pathways. Our recent work has provided mechanistic insights of how PD-1-TCR/CD3 co-ligation affects T cell signaling pathways that mediate induction of anergy, alters metabolic reprogramming and impacts the differentiation program of T effector cells. In the present exploratory application, we will test a novel hypothesis that co-ligating PD-1 and TCR is a novel approach to suppress aberrantly activated T cells. To this end, we will generate a single chain diabody termed PD1-scDb to co-engage PD-1 and TCR/CD3 to suppress T cell activation. This conceptually and technically novel approach is in stark contrast to the mainstream current approaches, which aim to block PD-1-mediated inhibitory signaling with the goal to enhance T cell activation. To demonstrate the potential therapeutic application of our novel compound to tame aberrantly activated T cells we will test its ability to reverse the signaling, metabolic and functional aberrations of autoreactive T cells and ameliorate symptoms of systemic lupus erythematosus (SLE), the prototype autoimmune disease. For this purpose we generated a prototype hPD1-scDb that showed potent inhibition of T cell responses, providing proof-of-concept of our novel idea and feasibility of our technical innovation. In our present proposal, we will use hPD1-scDb to investigate the molecular and biochemical mechanisms of its function and its ability to suppress responses of autoreactive T cells from patients with SLE. We will also generate a mouse (m) PD1-scDb and investigate its immunosuppressive properties in vitro and in vivo using mouse models of SLE. To achieve our goals we will pursue the following specific aims: SA1. To determine the effects of mPD1-scDb ex vivo and in vivo using mouse models of SLE. SA2. To identify the effects of hPD1-scDb on T cells from patients with SLE.